The following description is made with reference to devices particularly configured for bicycles with curved handlebars (typical of racing bicycles); however, the control device described could also be applied to straight handlebars (typical of mountain bikes).
A bicycle is normally provided with two derailleurs, a front one associated with the crankset and a rear one associated with the sprocket assembly. In both cases, the derailleur engages the transmission chain moving it over gear wheels of different diameters and numbers of teeth, so as to obtain different transmission ratios; the derailleur, be it the rear one or the front one, is moved in one direction by a tension exerted by an inextensible cable that is normally sheathed (commonly known as Bowden cable), and in the opposite direction by the elastic return action of an elastic return member (typically a spring), provided in the derailleur itself. Normally, the derailleur is moved by the elastic return member when the chain passes from a gear wheel of larger diameter to a gear wheel of smaller diameter, i.e. so-called downward gearshifting; vice-versa, the tension of the control cable takes place during so-called upward gearshifting, in which the chain moves from a gear wheel of smaller diameter to a gear wheel of larger diameter. It should be noted that in a front derailleur downward gearshifting corresponds to the passage to a lower transmission ratio, whereas in a rear derailleur it corresponds to a higher transmission ratio.
The movement in the two directions of the control cable of a derailleur is obtained through a control device mounted so as to be easy to maneuver for the cyclist, i.e. normally on the handlebars, close to the hand grips where the brake lever for commanding the brake actuation cable of the front or rear wheel is also located. Control devices that allow the control of both a derailleur in two directions and of a brake are commonly known as integrated controls. By convention, the control device of the front derailleur and the brake lever of the front wheel are located in the vicinity of the left hand grip, and vice-versa, the control device of the rear derailleur and the brake lever of the rear wheel are located in the vicinity of the right hand grip.
In most integrated control devices, the control cable is actuated in tension or in release through winding and unwinding on a rotor element, commonly known as cable-winding bush, the rotation of which is controlled by the cyclist with suitable control levers.